9,720 research outputs found
Character of the reaction between molecular hydrogen and silicon dangling bond in amorphous SiO_2
The passivation by diffusing H2 of silicon dangling bond defects (E' centers,
induced by laser irradiation in amorphous SiO_2 (silica), is investigated in
situ at several temperatures. It is found that the reaction between E' center
and H_2 requires an activation energy of 0.38eV, and that its kinetics is not
diffusion-limited. The results are compared with previous findings on the other
fundamental paramagnetic point defect in silica, the non bridging oxygen hole
center, which features completely different reaction properties with H_2.
Besides, a comparison is proposed with literature data on the reaction
properties of surface E' centers, of E' centers embedded in silica films, and
with theoretical calculations. In particular, the close agreement with the
reaction properties of surface E' centers with H_2 leads to conclude that the
bulk and surface E' varieties are indistinguishable from their reaction
properties with molecular hydrogen.Comment: 15 pages, 3 figures, submitted to J. Phys. Chem.
Hydrogen-Related Conversion Processes of Ge-Related Point Defects in Silica Triggered by UV Laser Irradiation
The conversion processes of Ge-related point defects triggered in amorphous
SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the
interplay between the (=Ge•-H) H(II) center and the twofold coordinated
Ge defect (=Ge••). The former is generated in the post-irradiation
stage, while the latter decays both during and after exposure. The
post-irradiation decay kinetics of =Ge•• is isolated and found to
be anti-correlated to the growth of H(II), at least at short times. From this
finding it is suggested that both processes are due to trapping of radiolytic
H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is
preserved also under repeated irradiation: light at 4.7eV destroys the already
formed H(II) centers and restore their precursors =Ge••. This
process leads to repeatability of the post-irradiation kinetics of the two
species after multiple laser exposures. A comprehensive scheme of chemical
reactions explaining the observed post-irradiation processes is proposed and
tested against experimental data.Comment: 25 pages, 7 figures, submitted to Phys. Rev.
Bleaching of optical activity induced by UV Laser exposure in natural silica
We report experimental data on two types of natural silica, differing for
their OH content, irradiated with UV photons (4.66 eV) from a pulsed Nd:YAG
laser. Irradiation induces a reduction of the absorption band at 5.12eV and of
the associated emissions at 3.14eV and 4.28eV, ascribed to twofold coordinated
Ge (=Ge'') centers pre-existing in our samples. The bleaching is mainly due to
the post-irradiation conversion of =Ge'' into the paramagnetic H(II) center via
trapping of a H atom. Comparison with literature data points out the
peculiarities of silica with a low Ge concentration as regards UV induced
transformations.Comment: Accepted for publication on Journal of Non crystalline solid
Behavior of rod-like polyelectrolytes near an oppositely charged surface
The behavior of highly charged short rod-like polyelectrolytes near
oppositely charged planar surfaces is investigated by means of Monte Carlo
simulations. A detailed microstructural study, including monomer and fluid
charge distribution, and chain orientation, is provided. The influence of chain
length, substrate's surface-charge-density and image forces is considered. Due
to the lower chain-entropy (compared to flexible chains), our simulation data
show that rod-like polyelectrolytes can, in general, better adsorb than
flexible ones do. Nonetheless, at low substrate-dielectric-constant, it is
found that repulsive image forces tend to significantly reduce this
discrepancy.Comment: Updated results - 19 pages - 10 figs - RevTex
Unitary decoupling treatment of a quadratic bimodal CQED model
We consider a two-photon quantum model of radiation-matter interaction
between a single two-level atom and a degenerate bimodal high-Q cavity field.
Within this tripartite system the explicit construction of two collective
radiation modes, one of which freely evolving and the other one quadratically
coupled to the matter subsystem, is reported. Meaning and advantages of such a
decoupling treatment are carefully discussed.Comment: accepted for publication in Physica Script
Influence of hydrogen on paramagnetic defects induced by UV laser exposure in natural silica
Diffusion limited reactions of point defects were investigated in amorphous
SiO2 exposed to UV laser light. Electron spin resonance and in situ absorption
measurements at room temperature evidenced the annealing of E' centers and the
growth of H(II) centers both occurring in the post-irradiation stage and
lasting a few hours. These transients are caused by reactions involving
molecular hydrogen H2, made available by dimerization of radiolytic H0.Comment: Submitted to Physica Status Solid
Zeno Dynamics and High-Temperature Master Equations Beyond Secular Approximation
Complete positivity of a class of maps generated by master equations derived
beyond the secular approximation is discussed. The connection between such
class of evolutions and physical properties of the system is analyzed in depth.
It is also shown that under suitable hypotheses a Zeno dynamics can be induced
because of the high temperature of the bath.Comment: 9 pages, 2 figure
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
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